The mu opiate receptor (OR) has been identified as the principal brain receptor site best correlated with the rewarding and euphoric properties of opiate drugs. This year investigators in this Branch have continued with vastly-reduced work with mu receptor knockout mice and phosphoregulation of the muOR due to personnel attenuation. Work on mu knockout mice during this year continued to document modest effects of mu knockout on several intersting features, including the relative robustness of the mu knockout effects to genetic background and effects of mu deletion on reward phenotypes from more and more drugs. During this year these investigators identified mu receptor region haplotypes associated with addiction vulnerability in several different human ganome scanning samples.